So, wait, let me get this straight, see if I understand this.
You build a bonfire workshop, and add in materials which are burned, in addition to wood. A dwarf is then chosen to 'set' it. You then have the dwarf who sets the bonfire run away.
So, for the next season or so, that 'workshop' will constantly set fire to everything around it? Is this speading out like a forest fire? Does this melt ice walls inside of glaciers, causing caveins? How precisely does this work?
Forest fires require grass - but yes, it will ignite items adjacent to and on top of the Bonfire workshop. For maximum security, putting a 3x3 cylinder of constructed wall or floor around the workshop allows you to dump any unwanted items directly onto the workshop and prevents it from igniting grass, since the target tiles are occupied. It is best to pre-build this wall and leave a diagonal corner open, so your dwarves have no chance to stand on the burning tile while making the wall.
I haven't done extensive testing for ice relations, but I can confirm that with a magma-safe material, the workshop can act as a water annihilator/steam producer. However, the smoke may interfere with this... but, as mentioned before, a non-burning item would be permanent, and therefore somewhat problematic.
Interestingly, these tiles will be designated as Warm tiles if they are natural. I'd go on to wager that one could melt a square of ice one floor above the Bonfire, though this warrants testing. If so, then heated pools may well be possible.